This invention relates to an apparatus for gas chromatography, and particularly, to an inlet system for such an apparatus.
Gas chromatography is a widely employed technique for the separation and analysis of complex mixtures of volatile organic and inorganic compounds. The analyte mixture is separated into its components by eluding them from a column having a sorbent by means of moving gas.
Gas chromatography procedures can be classified into two major divisions; gas-liquid chromatography, and gas-solid chromatography. Gas-liquid chromatography is presently the most widely employed type and incorporates a nonvolatile liquid sorbent coated as a thin layer on an inner support structure, generally the inside surface of a capillary tube. The moving gas phase, called the carrier gas, flows through the chromatography column. The analyte partitions itself between the moving gas phase and the sorbent, and moves through the column at a rate dependent upon the partition coefficient or solubility of the analyte components. The analyte is introduced at the entrance end of the column within the moving carrier gas stream. The components making up the analyte become separated along the column and escape from the exit end of the column at intervals and in concentrations characteristic of the properties of the analyte components. A detector, for example, a thermal conductivity detector or a flame ionization detector (FID) at the exit end of the column responds to the presence of analyte components. Upon combustion of the eluded material at the FID, charged species are formed in the flame. The flame behavior is monitored through a biased ion detector which, along with associated electronics, produces a chromatogram which is a time versus magnitude trace of the detector output. The trace for a complex mixture includes numerous peaks of varying intensity. Since individual constituents of the analyte produces peaks at characteristic times and whose magnitude is a function of their concentration, much information is gained through an evaluation of the chromatogram.
Gas chromatography systems of the type described above are in widespread use today. Although present systems provide excellent performance and utility, this invention seeks to provide improvements in gas chromatography systems; principally through simplifying the systems and increasing their speed and operational flexibility. Conventional gas chromatography system employs a mechanical system for the injection of analyte. For example, mechanical valves, or needles and septum type injection techniques are presently used. Such mechanical techniques contribute to system complexity, both in terms of their presence in the system and their control requirements. Conventional gas chromatography apparatuses are also unsatisfactory for high-speed analysis, since the column injection bandwidths are excessively large.
In accordance with the present invention, a gas chromatography system is provided in which no mechanical barriers are used to control the flow of analyte into the system. Instead, a thermal focusing chamber is provided at sub-ambient temperatures which is used as the exclusive means for controlling flow of analyte components into the separation column. The analyte stream continually flows into the thermal focusing chamber. Its introduction into the separation column, however, is controlled by the temperature of the sample tube in the thermal focusing chamber.
The injection system of this invention allows the high speed, repetitive sampling of a continuously flowing sample stream. With this invention, relatively simple mixtures can be separated faster than with current commercial apparatuses. Since the system has no moving parts, it is very rugged, and can operate for many cycles with minimal maintenance.
The flexibility of the inlet system according to this invention is demonstrated by the variety of optional modes it supports. In addition to high-pressure inlet operation with the column outlet at atmospheric pressure, the system is also capable of ambient and sub-ambient pressure inlet with vacuum outlet, and with vacuum backflush features. All of these modes of operation can be computer controlled.
This invention provides potential applications for numerous gas chromatography procedures, including those practiced by process engineers and chemists, industrial hygiene workers, and others interested in the continuous monitoring of volatile organic mixtures.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.